A petroleum reservoir is a subsurface pool of hydrocarbons (e.g., oil or gas) trapped in a subsurface rock formation. Oil and gas wells are often drilled into petroleum reservoirs to extract (or “produce”) the trapped hydrocarbons. It can be beneficial to understand the characteristics of a well when making critical decisions regarding completion and production of the well. For example, characteristics of the reservoir, such as reservoir saturation, can be used to determine whether the formation contains hydrocarbons, to estimate an amount of hydrocarbons or water in the formation, to predict the ability to produce the hydrocarbons, to determine optimal techniques for drilling and producing the hydrocarbons via the well, and so forth. Moreover, characteristics of the components of the well, such as the structural integrity of the well's casing, casing cement, and the like can be used to determine how the well is performing and what, if any, operations are needed to improve the performance of the well.
Well characteristics can be determined using a variety of different techniques. For example, certain well characteristics can be determined via coring or logging operations. Coring operations include the physical extraction of rock samples for assessment at the surface. Logging operations are performed to assess the formation and other components of the well down-hole. Logging operations typically include lowering one or more logging tools into a wellbore of a well, and recording measurements as the tool traverses the wellbore. This can be accomplished via wireline logging, logging-while-drilling (LWD), measurement-while-drilling (MWD), and/or the like. A plot of the measurements versus depth is typically referred to as a “log”. Logs can be analyzed to determine characteristics of the well and the reservoir, such as reservoir water saturation.
There are many types of logging available, and a particular form of logging may be selected and used based on the logging conditions and the type of measurements to be acquired. For example, pulsed neutron (PN) logging operations measure absorption of neutrons by the wellbore and the reservoir area around a wellbore. This absorption measurement can be used to determine characteristics of the well and the reservoir, such as water saturation (Sw) of the reservoir. PN logging tools often employ a source to emit bursts of high energy neutrons that are absorbed by nuclei in the wellbore and the formation around the wellbore, and detectors to measure gamma rays output by the nuclei as a result of absorbing the neutrons. The gamma ray population is observed to decay for each burst to determine absorption of the neutrons, and the determined absorption can be used to generate corresponding water saturation values and a water saturation log for the well. As another example, acoustic based cement-bond (CB) logging measures acoustic amplitude attenuation. This amplitude attenuation measurement can be used to determine characteristics of the well, such as a degree of coupling of cement to the casing and the formation. CB logging tools often employ one or more acoustic transmitters to emit an acoustic wave that propagates into the casing, the cement and the formation, and two or more acoustic receivers to measure resulting acoustic waves. Characteristics of the resulting acoustic waves, such as amplitude, can be used to determine the structural integrity of the cement, including its bond to the casing and the formation.